U.S. patent number 4,851,775 [Application Number 07/069,495] was granted by the patent office on 1989-07-25 for digital compass and magnetometer having a sensor coil wound on a high permeability isotropic core.
This patent grant is currently assigned to Precision Navigation, Incorporated. Invention is credited to Timothy Hawks, Nam H. Kim.
United States Patent |
4,851,775 |
Kim , et al. |
July 25, 1989 |
Digital compass and magnetometer having a sensor coil wound on a
high permeability isotropic core
Abstract
A digital compass (20) has a sensing coil (60) wound on an
elongated strip of high direct current permeability magnetic
material. The sensing coil (60) is connected to a sensing circuit
(56). The sensing coil and sensing circuit are responsive to the
Earth's magnetic field to provide an oscillating signal at an
output (28) of the sensing circuit (56) which varies in frequency
with orientation of the at least one sensing coil (60) with respect
to the Earth's magnetic field. A microprocessor (36) is connected
to receive information inputs from the oscillating signal. The
microprocessor converts the information inputs to an indication of
orientation of the sensing coil with respect to the Earth's
magnetic field based on the frequency of the oscillating signal. A
display (52) receives the orientation indication from the
microprocessor.
Inventors: |
Kim; Nam H. (Glendale, CA),
Hawks; Timothy (Stanford, CA) |
Assignee: |
Precision Navigation,
Incorporated (Menlo Park, CA)
|
Family
ID: |
22089373 |
Appl.
No.: |
07/069,495 |
Filed: |
July 2, 1987 |
Current U.S.
Class: |
324/247;
324/260 |
Current CPC
Class: |
G01C
17/28 (20130101); G01R 33/02 (20130101) |
Current International
Class: |
G01C
17/00 (20060101); G01C 17/28 (20060101); G01R
33/02 (20060101); G01R 033/02 () |
Field of
Search: |
;324/244,247,249,252,253,254,255,256,260 ;331/65 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Snow; Walter E.
Attorney, Agent or Firm: Smith; Joseph H.
Claims
What is claimed is:
1. A magnetometer comprising:
an oscillator circuit comprising:
oscillator driver means having first and second terminals for
providing an oscillating current; and
sensor means having a first coil wound on a first high permeability
isotropic core for controlling the frequency of said oscillator
circuit, said first coil coupled between said first and second
terminals, said first coil having a magnetic axis;
said oscillator driver means for providing a DC bias current
through said first coil, such that when said sensor means
experiences a change in an externally applied magnetic field, the
frequency of the oscillating current changes monotonically with the
change in magnitude of the applied external magnetic field in the
direction of the magnetic axis of said first coil; and
measurement means for measuring the frequency of the oscillating
current and providing a measurement signal in response thereto.
2. A magnetometer as in claim 1 wherein said oscillator driver
means includes resistive means connected between a source of an
electric potential and said sensor means for providing said DC bias
current.
3. A magnetometer as in claim 1 wherein said measurement means
comprises microprocessor means for providing said measurement
signal such that said measurement signal is functionally related to
the magnitude and sign of the externally applied magnetic
field.
4. A magnetometer as in claim 3 further comprising display means
coupled to receive said measurement signal from said microprocessor
means for displaying a representation of the magnitude and sign of
the externally applied magnetic field relative to the magnetic axis
of said first coil.
5. A magnetometer as in claim 1 further comprising:
a second oscillator circuit comprising:
second oscillator driver means having first and second terminals
for providing a second oscillating current; and
second sensor means having a second coil wound on a second high
permeability isotropic core for controlling the frequency of said
second oscillator circuit, said second coil coupled between said
first and second terminals of said second oscillator driver means,
said second coil having a magnetic axis with a directional
component orthogonal to the magnetic axis of the first coil;
said second oscillator driver means for providing a DC bias current
through said second coil, such that when said second sensor means
experiences a change in the externally applied magnetic field, the
frequency of the second oscillating current changes monotonically
with the change in magnitude of the applied external magnetic field
in the direction of the magnetic axis of said second coil; and
wherein said measurement means includes means for measuring the
frequency of the second oscillating current and providing a second
measurement signal in response thereto.
6. A magnetometer as in claim 5 further comprising:
a third oscillator circuit comprising:
third oscillator driver means having first and second terminals for
providing a third oscillating current; and
third sensor means having a third coil wound on a third high
permeability isotropic core for controlling the frequency of said
third oscillator circuit, said third coil coupled between said
first and second terminals of said third oscillator driver means,
said third coil having a magnetic axis with a directional component
orthogonal to the magnetic axis of the first coil and the magnetic
axis of the second coil;
said third oscillator driver means for providing a DC bias current
through said third coil, such that when said third sensor means
experiences a change in the externally applied magnetic field, the
frequency of the third oscillating current changes monotonically
with the change in magnitude of the applied external magnetic field
in the direction of the magnetic axis of said third coil; and
wherein said measurement means includes means for measuring the
frequency of the third oscillating current and providing a third
measurement signal in response thereto.
7. A magnetometer as in claim 1 wherein said high permeability
material is a metallic glass alloy.
8. A magnetometer as in claims 6 wherein said first, second and
third oscillator drivers comprise Schmitt trigger circuits.
9. A digital compass comprising:
a first voltage comparator circuit having first and second input
terminals, and a output terminal, said output terminal being
coupled back to said second input terminal and said second input
terminal being coupled to a first reference potential;
a first sensing coil wound on a high permeability isotropic core
having a magnetic axis and being connected between said first input
terminal and said output terminal of said first voltage comparator
circuit so as to form a first relaxation oscillator, said first
relaxation oscillator providing a first oscillating signal having a
frequency that is a monotonic function of the magnitude of the
earth's local magnetic field in the direction of the magnetic axis
of the first sensing coil; and
processor means coupled to receive said oscillating signal from
said first relaxation oscillator for providing an indicator signal
representative of the orientation of the magnetic axis of said
first sensing coil with respect to the direction of the earth's
local magnetic field.
10. A digital compass as in claim 9 further comprising display
means coupled to receive said indicator signal for displaying a
signal related to the relative orientation of said first sensing
coil with respect to the direction of the earth's local magnetic
field.
11. A digital compass of claim 9 wherein:
said magnetic axis of said first sensing coil is the X axis; said
digital compass further comprises:
a second voltage comparator circuit having first and second input
terminals, and an output terminal, with the second input terminal
being connected to a second reference potential;
a second sensing coil wound on a high permeability isotropic core
having a magnetic axis, the Y axis, that is orthogonal to the X
axis, said second sensing coil being connected between said first
input terminal and said output terminal of said second relaxation
oscillator, said second relaxation oscillator providing a second
oscillating signal having a frequency that is a monotonic function
of the magnitude of the earth's local magnetic field in the
direction of the magnetic axis of the second sensing coil;
a third voltage comparator circuit having first and second input
terminals, and an output terminal, with the second input terminal
being connected to a third reference potential; and
a third sensing coil wound on a high permeability isotropic core
having a magnetic axis, the Z axis, that is orthogonal to both the
X and the Y axes, said third sensing coil being connected between
said first input terminal and said output terminal of said third
voltage comparator circuit so as to form a third relaxation
oscillator, said third relaxation oscillator providing a third
oscillating signal having a frequency that is a monotonic function
of the magnitude of the earth's local magnetic field in the
direction of the magnetic axis of the third sensing coil; and
said processor means further coupled to receive said oscillating
signals from said second and third relaxation oscillators for
providing indicator signals representative of the orientation of
the magnetic axis of said second and third sensing coils with
respect to the direction of the earth's local magnetic field.
12. The digital compass of claim 11 wherein at least one of said
first, second, and third voltage comparator circuits is a Schmitt
trigger circuit.
13. A digital compass of claim 9 wherein said high permeability
material is a metallic glass alloy.
14. A digital compass comprising the magnetometer of claim 6 and
wherein said applied external magnetic field is the earth's
magnetic field.
15. The digital compass of claim 9 wherein:
said magnetic axis of said first sensing coil is the X axis; said
digital compass further comprising:
a second voltage comparator circuit having first and second input
terminals, and a output terminal, said output terminal of said
second voltage comparator circuit being coupled back to said second
input terminal of said second voltage comparator circuit;
a second sensing coil having a magnetic axis, the Y axis, that is
orthogonal to the X axis, said second sensing coil being connected
between said first input terminal and said output terminal of said
second voltage comparator circuit so as to form a second relaxation
oscillator, said second relaxation oscillator providing a second
oscillating signal having a frequency that is a monotonic function
of the magnitude of the earth's local magnetic field in the
direction of the magnetic axis of the second sensing coil; and
said processor means further coupled to receive said second
oscillating signal from said second relaxation oscillator for
providing an indicator signal representative of the orientation of
the magnetic axis of said second sensing coil with respect to the
direction of the earth's local magnetic field.
16. The digital compass of claim 15 wherein at least one of said
first and second voltage comparator circuits is a Schmitt trigger
circuit.
17. The digital compass of claim 15 wherein said high permeability
cores are constructed of a metallic glass alloy.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a digital compass in which direction of
the Earth's magnetic field is determined on the basis of frequency
differences which are a function of the orientation of the digital
compass with respect to the Earth's magnetic field. More
particularly, it relates to such a digital compass in which it is
not necessary to convert analog signals to digital signals. It
further relates to a novel form of a magnetometer which has general
application for determining orientation of weak magnetic fields
based on frequency differences.
2. Description of the Prior Art
A variety of digital compasses and magnetometers are known in the
art. For example, U.S. Pat. No. 3,396,329, issued Aug. 6, 1968 to
Salvi, discloses a magnetometer in which the intensity of weak
magnetic fields is a function of frequency difference in sensed
signals, but independent of orientation of a vessel in which the
magnetometer is installed. U.S. Pat. No. 3,634,946, issued Jan. 18,
1972 to Star, relates to an all digital circuit implementation of a
digital compass which operates on the basis of spatial
relationships of pulses produced when a sensor is aligned in a
reference direction and orthogonal to the Earth's magnetic field.
There is no mention in this patent of frequency differences created
by orientation, nor does the circuit shown discriminate on the
basis of such frequency differences. U.S. Pat. No. 4,305,034,
issued Dec. 8, 1981 to Long et al., discloses a magnetometer in
which frequency changes are created when a background magnetic
field, which can be the Earth's magnetic field, is perturbed by a
metal object, but this device cannot provide sign information,
i.e., whether the field is parallel or antiparallel to the sensor
coil. U.S. Pat. No. 4,340,861, issued July 20, 1982 to Sparks,
discloses a magnetometer in which frequency differences are used to
determine distribution of magnetic fields produced by permanent
magnets, on the basis of amplitude information in the different
frequency signals. U.S.S.R. Pat. No. 945,835, issued July 27, 1982
to Bondarevsk et al., discloses that a strong magnetic field will
produce frequency differences in an LC circuit.
The following additional issued U.S. patents relate to digital
compasses which utilize phase differences, comparison with previous
signals at known orientations or counting of sensing marks to
determine orientation: No. 3,490,024, issued Jan. 13, 1970 to
Sherrill et al.; No. 3,903,610, issued Sept. 9, 1970 to Heaviside
et al.; No. 3,952,420, issued Apr. 27, 1976 to Benjamin et al.; No.
4,095,348, issued June 20, 1978 to Kramer; No. 4,179,741, issued
Dec. 18, 1979 to Rossani; No. 4,424,631, issued Jan. 10, 1984 to
Franks and No. 4,640,016, issued Feb. 3, 1987 to Tanner et al. The
following issued U.S. patents relate generally to magnetometers:
No. 3,432,751, issued Mar. 11, 1969 to Godby et al.; No. 3,435,337,
issued Mar. 25, 1969 to Inouye et al., No. 3,461,387, issued Aug.
12, 1969 to Morris et al., No. 3,768,011, issued Oct. 23, 1973 to
Swain and No. 4,641,094, issued Feb. 3, 1987 to Dalton, Jr. The
state of the art in magnetometer design is further indicated by
Takeuchi et al., "A Resonant-Type Amorphous Ribbon Magnetometer
Driven by an Operational Amplifier," IEEE Transactions on
Magnetics, Vol. MAG-20, No. 5, September 1984, pp. 1723-1725.
While the art relating to the design of digital compasses and
magnetometers is thus a well-developed one, a need remains for
development of a simple, reliable, low cost digital compass
suitable for consumer use and a simple magnetometer for determining
orientation of low intensity magnetic fields.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a digital
compass which can be implemented with a simple digital circuit and
which is sufficiently low cost for consumer applications.
It is another object of the invention to provide such a digital
compass in which orientation with respect to the Earth's magnetic
field is determined by frequency differences obtained with a
sensing circuit.
The attainment of these and related objects may be achieved through
use of the novel digital compass herein disclosed. A digital
compass in accordance with this invention has at least one sensing
coil wound on an elongated strip of high direct current
permeability magnetic material. The sensing coil is connected to a
sensing circuit. The at least one sensing coil and sensing circuit
are responsive to the Earth's magnetic field to provide an
oscillating signal at an output of the sensing circuit which varies
in frequency with orientation of the at least one sensing coil with
respect to the Earth's magnetic field. A microprocessor is
connected to receive information inputs from the oscillating
signal. The microprocessor is configured to convert the information
inputs to an indication of orientation of the at least one sensing
coil with respect to the Earth's magnetic field based on the
frequency of the oscillating signal. A display means is connected
to receive the orientation indication from the microprocessor.
The frequency of the oscillating signal at the output of the
sensing circuit varies substantially, e.g., by about 100 percent,
as the sensing coil is moved from a parallel to an antiparallel
orientation with respect to the Earth's magnetic field. Such
substantial frequency differences mean that a very accurate digital
readout of angle between the sensing coil orientation and magnetic
North is obtained from the microprocessor.
Similarly, a magnetometer in accordance with the invention has at
least one sensing coil wound on an elongated strip of high direct
current permeability magnetic material. The sensing coil is
connected to a sensing circuit. The at least one sensing coil and
sensing circuit are responsive to a magnetic field to provide an
oscillating signal at an output of the first sensing circuit which
varies in frequency with orientation of the at least one sensing
coil with respect to the magnetic field. The sensing coil is
connected to be self-biased by a direct current through the sensing
coil. A means for measuring a frequency of the oscillating signal
and providing an indication of the frequency is connected to
receive the oscillating signal.
The attainment of the foregoing and related objects, advantages and
features of the invention should be more readily apparent to those
skilled in the art, after review of the following more detailed
description of the invention, taken with the drawings, in
which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a hysteresis curve for a sensing element used in a
digital compass in accordance with the invention.
FIG. 2 is a plot useful for understanding operation of the
invention.
FIG. 3 is a schematic diagram of a sensing circuit used in a
digital compass in accordance with the invention.
FIG. 4 is a block diagram of a digital compass in accordance with
the invention.
DETAILED DESCRIPTION OF THE INVENTION
Turning now to the drawings, more particularly to FIG. 1, there is
shown a hysteresis curve 10 for an isotropic material such as
METGLAS Amorphous Alloy 2705 M, obtainable from Allied Signal
Corporation. This material is a cobalt-based magnetic alloy which
is characterized by near-zero magnetostriction and high
DC-permeability without annealing. This material is used to form
sensors for the digital compass of this invention by winding a coil
around a straight strip of the alloy in a traditional solenoid
geometry.
The following background information will facilitate an
understanding of the invention. For magnetic core solenoids, the
following equations are generally true.
where H is the magnetizing force, n is the turn density of the coil
in turns per unit length, .mu..sub.0 is the permeability of free
space, and I is the current flowing in the coil.
where E is the potential across the coil in volts, n is the turn
density in turns per unti length, V is the volume of the core
material, and dB/dt is the time derivative of the total magnetic
flux.
For small transitions or changes in H, the coil can be modeled as
an ideal inductor, where
By substitution of the previous equations and by solving, the
following can be shown
where dB/dH is the slope of the B vs. H curve at a particular
point.
Let .mu.(H)=dB/dH. While most magnetic core materials exhibit a
constant .mu. over a large range of H, the above METGLAS alloy has
uniquely different characteristics, as shown by the .mu.(H) plot of
FIG. 2. As shown, by providing a dc bias current through the coil,
producing a magnetizing force H.sub.0, the coil can be biased at an
operating point which is in the middle of sloping region 12 of
.mu.(H) curve 14. A static magnetic field parallel to the coil will
be added to the H.sub.0 and will move the operating point in either
direction dependent on the polarity of the static magnetic field.
Since the inductance L is proportional to .mu.(H), the inductance
will change appreciably with the applied static magnetic field.
The above principles of induction variation can be observed with a
relaxation oscillator sensing circuit 16 using a Schmitt trigger 18
as shown in FIG. 3. The period of the output, T, is proprtional to
L/R. The dc bias current is dependent on R and the threshold levels
of the Schmitt trigger 18. In general, by using the sensor as the
inductor timing element of any oscillator circuit, a change in the
static field will produce a frequency change at the output. The
duty cycle should be asymmetrical and will not vary significantly
with the operating point in the linear region .mu.(H), i.e., the
sloping portion 12 of the curve 14 in FIG. 2. Such a frequency
change detection scheme renders an analog to digital (A/D)
converter unnecessary. Note that the linearity of the .mu.(H)
region is not essential to recover useful information; the
operation region merely has to be monotonic.
FIG. 4 shows a digital compass 20 utilizing a sensing circuit 23 of
the type shown in FIG. 3. The compass 20 has X, Y and Z sensors 23,
24 and 26 respectively connected by lines 28, 30 and 32 to an
interface circuit 34. Interface circuit 34 is connected to
microprocessor 36 by line 38. The microprocessor 36 is connected to
a read only memory (ROM) 40 and to a random access memory (RAM) 42
by lines 44 and 46, respectively. The microprocessor 36 is
connected to a display driver 48 by line 50. Display driver 48 is
in turn connected to display 52 by line 54.
The X, Y and Z sensors 23, 24 and 26 each have the configuration
shown for the X sensor 23. The X sensor 23 has a Schmitt trigger
circuit 56 implemented with a LM 339 type voltage comparator
integrated circuit, obtainable from National Semiconductor
Corporation, Santa Clara, Calif. A +Vcc input is connected through
a 50K ohm variable resistor R1 to the positive input of the Schmitt
trigger 56 by line 58. A sensor coil 60 having 1200 turns of wire
around a straight strip of METGLAS Amorphous Alloy 2705 M with a
length of 1.8 cm, a width of 0.5 mm and a thickness of 20 .mu.m is
connected to the negative input of the Schmitt trigger 56 by line
62. A +Vcc input is also connected through a 5K variable resistor
R2 to the negative input of the Schmitt trigger 56. The output of
the Schmitt trigger 56 is connected by the line 28 to the interface
circuit 34. The output of the Schmitt trigger is also fed back on
line 64 through the sensor coil 60 to the input. The output is also
connected to +Vcc through a 1N4148 type diode D1, and fed back on
line 66 through a 4.7K resistor R3 to the positive input of the
Schmitt trigger 56. The resistor R2 can be used to adjust both the
bias current (and hence the operating point) and the frequency of
the oscillator. R1 will change the location of the Schmitt
trigger's positive and negative thresholds. R3 can be used to
adjust the frequency and the current swing of the oscillator
circuit.
In operation, as noted above, the period T of the oscillating
output of the Schmitt trigger 56 is proportional to L/R at the
input. The value of L varies with the orientation of the sensor
coil 60 with respect to the Earth's magnetic field. Where He.sub."
is the component of the Earth's magnetic field parallel to the
length of the sensor 60 and He.sub." is taken to be positive along
the direction of H.sub.0, He.sub." can be very precisely determined
by detecting frequency deviation. By having two sensors in
orthogonal directions, such as x and y, .theta., the orientation
angle of the magnetic North with respect to the fixed direction of
the compass 20 can be determined according to the formula
By having three sensor 23, 24 and 26, the orientation angle of
magnetic North can be determined at any fixed direction of the
compass 20 in three dimensions. With inclination information, we
extract the two components He.sub." y and He.sub." x, which are
parallel to the Earth's surface.
In practice, an oscillating center frequency f.sub.0 of about 200
kHz is obtained with the sensors 23, 24 and 26. A frequency change
of about 100% is obtained as one of the sensors 23, 24 and 26 is
rotated from a parallel to an antiparallel direction with respect
to the Earth's magnetic field. This magnitude of frequency change
gives very accurate digital read out of orientation with the
digital compass 20.
It should now be readily apparent to those skilled in the art that
a novel digital compass capable of achieving the stated objects of
the invention has been provided. The digital compass of this
invention uses a simple digital circuit and is therefore of
sufficiently low cost for consumer applications. The compass
determines orientation with respect to the Earth's magnetic field
based on frequency differences as the direction of a sensor changes
with respect to the Earth's magnetic field. The sensor produces
large enough frequency differences so that a very accurate digital
read out of orientation is obtained.
It should further be apparent to those skilled in the art that
various changes in form and details of the invention as shown and
described may be made. It is intended that such changes be included
within the spirit and scope of the claims appended hereto.
* * * * *